Stencil Stamp not showing up
Two steps would help to solve what happened in your case:


Set the Alpha percentage of Overlay to some non-zero-percent value;
Press Reset Transform button right behind the texture selector.


As shown below.



Btw, your texture stencil is on the upperleft corner of the 3D Viewport, which can be seen if you maximize the 3D view.

Shortcut tips when working with texture stencil and texture mask stencil:


For Texture Stencil:


RMB: Stencil translation

CtrlRMB: Stencil rotation

ShiftRMB: Stencil scale (Allow constraining on X/Y axis)


For Texture Mask Stencil:


AltRMB: Stencil translation

AltCtrlRMB: Stencil rotation

AltShiftRMB: Stencil scale (Allow constraining on X/Y axis)